1. Field of the Invention
The invention relates to composite structures and, more particularly to composite structures with thermal barrier coatings of zirconia stabilized with scandia and yttria.
2. Description of the Related Art
Thermal barrier coatings (TBCs) are widely used in the gas turbine engine industry to protect metal engine components such as combustion chambers and turbine blades from high temperatures and thereby to increase engine component life and to improve engine efficiency. Because engine power and efficiency increase with gas operating temperatures, the search is ongoing for metal and coating compositions that can withstand higher temperatures.
Zirconia (ZrO.sub.2) is a ceramic that has excellent heat insulating properties that are desirable in a thermal barrier coating. However, pure or unstabilized zirconia undergoes a catastrophic tetragonal-to-monoclinic phase change when subjected to thermal cycling through the range of 1000.degree. C.-1100.degree. C. This phase transformation results in a change in volume of zirconia and can cause disastrous flaking and deterioration of a thermal barrier coating containing zirconia. To avoid the phase transformation and consequent flaking and deterioration, zirconia can be combined with stabilizing oxides such as yttria (Y.sub.2 O.sub.3), calcia (CaO), magnesia (MgO), ceria (CeO.sub.2), scandia (Sc.sub.2 O.sub.3) (see U.S. Pat. No. 4,913,961, incorporated herein by reference in its entirety) and india (In.sub.2 O.sub.3) (see U.S. Pat. Nos. 5,288,205; 5,418,060 and 5,532,057, each incorporated herein by reference in its entirety). When zirconia containing a stabilizing oxide such as yttria, scandia or india is heated into the cubic or liquid phase and quenched rapidly, a metastable tetragonal t' phase is formed that resists tetragonal-monoclinic transformations during subsequent thermal cycling. The stabilized zirconia can then be used in a thermal barrier coating. The most commonly used stabilizing oxide for thermal barrier coatings is yttria, which is commonly added to zirconia in the amount of 6-8 wt %.
A disadvantage of yttria-stabilized zirconia in thermal barrier coatings is that the protective t' phase breaks down at ultra-high temperatures (temperatures above 1200.degree. C.). Equilibrium tetragonal t and cubic f phases are formed at these temperatures, and upon cooling, the tetragonal t phase transforms to the monoclinic m phase much the same as it does in unstabilized zirconia. This phase instability of yttria-stabilized coatings at ultra-high temperatures is and will continue to be a major drawback, especially as high efficiency engines are developed that operate with coating surface temperatures of 1400.degree. C. or higher.
A further disadvantage of yttria-stabilized zirconia in thermal barrier coatings is that the coatings are easily damaged by exposure to hot corrosive fuel contaminants such as vanadium-and sulfur-containing compounds. Such exposure may happen, for example, in wartime when high qualtity fuel is scarce. When the yttria in a yttria-stabilized zirconia thermal barrier coating reacts with vanadium impurities in fuel, the coating becomes yttria-depleted and destabilized.
Scandia-stabilized zirconia has been tested as an alternative to yttria-stabilized zirconia in thermal barrier coatings. As described in U.S. Pat. No. 4,913,961, scandia-stabilized zirconia is much more resistant to damage from exposure to vanadium- and sulfur-containing compounds than yttria-stabilized zirconia.
In the field of thermal barrier coatings, improvements in the performance of a coating can have a large economic impact by allowing more efficient gas turbine engines to be designed and built. Therefore, it would be desirable to have a thermal barrier coatings with even better phase stability at ultra-high temperatures than what can be obtained with yttria-stabilized zirconia or scandia-stabilized zirconia.